[GitHub] [spark] srowen commented on a change in pull request #24709: [SPARK-27841][SQL] Improve UTF8String to/fromString()/numBytesForFirstByte() performance

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srowen commented on a change in pull request #24709: [SPARK-27841][SQL] Improve UTF8String to/fromString()/numBytesForFirstByte() performance
URL: https://github.com/apache/spark/pull/24709#discussion_r287609636
 
 

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 File path: common/unsafe/src/main/java/org/apache/spark/unsafe/types/UTF8String.java
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 @@ -1217,6 +1264,20 @@ public boolean toByte(IntWrapper intWrapper) {
 
   @Override
   public String toString() {
+    byte[] bytes = getBytes();
+    // Optimization for ASCII characters: use deprecated string API which
+    // skips charset encoder and simply casts each byte into a char.
+    if (isAscii(bytes)) {
+      return new String(bytes, 0);
 
 Review comment:
   I get that we might want to go off-road here and call a deprecated method, but does it help meaningfully? the benchmark doesn't have the resolution to determine it. Is it as fast to, maybe, pass US_ASCII as the charset in this case?

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